The present invention relates to a new and improved method of spraying or injecting concrete, and furthermore, pertains to novel apparatus for the performance of the aforementioned method.
Generally speaking, the method of spraying concrete as contemplated by the invention utilizes compressed air, wherein a dry mixture composed of cement and additives has added thereto water and at least part of the compressed air, serving as the conveying medium, is withdrawn from the conveying path before the sprayable material departs from a spray nozzle.
This type of method serves for application of concrete to a substrate, such as a wall or an arch, for instance in galleries, during tunnel construction and in mines. The compressed air serving as a conveying medium must have an adequate pressure in order to be able to convey the concrete mass through a conveying path from a spraying or injection machine up to the spraying or injection nozzle. If the concrete mass has incorporated therein an adequate quantity of water, already forwardly of such conveying path, then the volumetric weight of the concrete mass is greater than that of a dry concrete mass. Additionally, the friction in the conveying path increases owing to the tackiness of the mass, so that the compressed air must be at such a high pressure that there can be positively avoided clogging of the conveying hose serving as the conveying path.
Yet, the high pressure of the conveying air causes the concrete mass, emanating from the injection nozzle, to be propelled against the support surface or substrate with such a great force that an appreciable amount of the sprayed concrete mass rebounds or splatters back from the support surface or substrate and drops to the ground as unusable material and as undesirable waste. The economies of the spraying or injection method are appreciably impaired due to the loss of part of the concrete mass, since this waste proportion can be quite appreciable in terms of the entire amount of concrete mass which is to be sprayed.
On the other hand, if the concrete mass is conveyed up to the region of the spray nozzle in an almost dry state and the requisite quantity of water is first added at the location of the spray nozzle, then while there is adequate lesser pressure of the conveying air, nonetheless the time of action of the water at the concrete mass is too short, so that the adherence of the concrete mass at the support surface is rendered more difficult due to inadequate tackiness. Additionally, there is considerable dust formation which can appreciably impair visibility.
Moreover, the rebound of the concrete at the support surface is further augmented by the action of the compressed air which effluxes together with the concrete mass out of the spray nozzle, in that the concrete mass is again blow-off of the support surface. Therefore, it is known to the art to separate part of the compressed air from the concrete mass directly at or shortly ahead of the spray or injection nozzle and to outfeed such to the surrounding atmosphere. The withdrawal or branching-off of part of the compressed air occurs directly at the spray nozzle. One prior art construction of spray nozzle is curved and at the inside of the arc or curve has an air withdrawal opening. A pressure equalization between the interior of the spray nozzle and the surrounding atmosphere takes place through this opening. Consequently, the concrete mass flows, by virtue of the centrifugal force, along the outer surface of the curve or arc.
According to another proposal the nozzle is formed of two tubes. The outer tube is tapered in a funnel-shaped manner, whereas the inner tube opens into this funnel-like tube, similar to an injector. The funnel or funnel-like tube is open at the rear side, with respect to the spraying or injection direction, whereas its front side forms the mouth of the spray nozzle. The material jet is propelled through the funnel or nozzle mouth, owing to the high kinetic energy, whereas a part of the conveying air can escape towards the rear.
Notwithstanding the use of the previously described spray nozzles the concrete mass is propelled with the full kinetic energy upon the support surface or subtrate, producing the drawbacks which have already been discussed above. Furthermore, the spray nozzles designed according to the aforementioned proposals are both heavy and cumbersome to use.